1. Field of the Invention:
The present invention relates to a method and apparatus for making a semiconductor element and more particularly to a method and apparatus for making a semiconductor element with at least two junctions between regions of alternating opposite conduction types and different doping concentrations, and with the semiconducting regions enclosed between the junctions exhibiting a higher resistivity then either of the neighboring regions.
2. Description of the Prior Art:
Known semiconductor components with several PN junctions, e.g., thyristors, designed for high off-stage voltages in the range of a few kilovolts, have various fundamental drawbacks. Although known elements of this type, judging from their dimensions, could bear relatively high transient or steady off-state voltages, they must be operated at lower off-state voltages for reasons of safety. The reason is that premature voltage breakdowns are to be expected in the region of the surface edge of the junction. The instabilities to be expected frequently derive from the fact that the state of the semiconductor surface changes with time. The surface off-state currents resulting from voltage peaks are concentrated in a few microscopically small fissures. These fissures usually appear only at the surface edge of the junction. In such a narrow zone, however, the production of heat can be so concentrated as to cause melting, regardless of the size of the semiconductor element, and the destruction of the ability of the element to withstand off-state voltage. That voltage at which the blocking junction loses its high impedance is designated as its breakdown voltage in the blocking direction. From the Swiss Patent No. 414,866, it is known how to avoid such breakdowns by sloping the semiconductor surface between two neighboring junctions by means of a groove. The sand jet method proposed for making the groove seldom is successful, however, because the groove generally cannot be maintained truly parallel to the PN-junctions.